Paper may be viewed as a network of cellulosic fibers that are bonded together at those areas where the fibers come in contact with one another. As the bonded areas are made closer to one another, each fiber will have correspondingly less unbonded area. This situation may be brought about by the customary beating or refining techniques that shorten the fibers, increase their surface area, and make them more flexible or pliable while wet. If the fiber network is made up mostly of bonded areas, it may be expected to be less flexible than a network of fibers that has mostly unbonded areas.
It is well known that generally as the extent of fiber refining is increased, the tensile strength and bursting strength of the ultimately formed paper may increase but the tearing strength will decrease. In other words, a paper whose fibers are well bonded as a result of a high degree of refining of the pulp will have greater tensile strength and bursting strength than a paper having only moderately bonded fibers but it will be less resistant to tear. The well bonded paper transmits applied stress directly to the propagation point of a tear so that the paper tears more easily. On the other hand, a moderately bonded paper will tend to distribute the applied stress to areas adjacent to the tear propagation point as well as to the propagation point so that it has more resistance to tear. The papermaker is therefore faced with the choice of refining the fiber more so as to improve its tensile and bursting strengths or refining it less so as to retain as much resistance to tear as possible.